Haskell 2010 changes the syntax for [[guard]]s by replacing the use of a single condition with a list of qualifiers. These qualifiers, which include both conditions and ''pattern guards'' of the form ''pat <- exp'', serve to bind/match patterns against expressions. The syntax is comparable that of a [[list comprehension]], where instead the types of ''pat'' and ''exp'' match. This makes it easy, for instance, to pattern match against (possibly failing) table lookups while deciding which definition of a function to use.

Haskell 2010 changes the syntax for [[guard]]s by replacing the use of a single condition with a list of qualifiers. These qualifiers, which include both conditions and ''pattern guards'' of the form ''pat <- exp'', serve to bind/match patterns against expressions. The syntax is comparable that of a [[list comprehension]], where instead the types of ''pat'' and ''exp'' match. This makes it easy, for instance, to pattern match against (possibly failing) table lookups while deciding which definition of a function to use.

−

Stealing a function from the note,

+

From the GHC user's guide,

<haskell>

<haskell>

+

lookup :: FiniteMap -> Int -> Maybe Int

+

addLookup env var1 var2

addLookup env var1 var2

| Just val1 <- lookup env var1

| Just val1 <- lookup env var1

Revision as of 20:48, 9 May 2013

Haskell 2010 changes the syntax for guards by replacing the use of a single condition with a list of qualifiers. These qualifiers, which include both conditions and pattern guards of the form pat <- exp, serve to bind/match patterns against expressions. The syntax is comparable that of a list comprehension, where instead the types of pat and exp match. This makes it easy, for instance, to pattern match against (possibly failing) table lookups while deciding which definition of a function to use.